The overall objective of these studies is to investigate the carcinogenicity of asbestos fibers in mice and rats, following chronic oral administration to pregnant females, and to F1 progeny for life. Groups of pregnant CD rats and CF mice (Charles River) were orally administered with chrysotile and amosite fibers at two different concentrations (12.5 mg/kg and 50 mg/kg; 5 doses each), and then the newborns are treated with the same dosage of the fiber as their mothers at a twice-weekly schedule to be continued for life. These studies are in progress.